Ultrasonic flowmeters are commonly used to determine the flow rate for a variety of fluids (e.g., liquids, gases) as well as the speed of sound in the fluid flowing in pipes having a variety of different sizes (e.g., 4-inch to 24-inch) and shapes. Knowledge of the flow rate of the fluid can enable other physical properties or qualities of the fluid to be determined. For example, in some custody-transfer applications, the flow rate can be used to determine the volume (Q) of a fluid (e.g., oil or gas) being transferred from a seller to a buyer through a pipe to determine the cost for the transaction, where the volume is equal to the flow rate multiplied by the cross-sectional area (A) of the pipe. In other applications, the speed of sound can be used to determine the mean molecular weight of a fluid flowing in a pipe to improve and/or control a chemical process or a combustion processes.
One type of ultrasonic flowmeter employs transit time flow metering, where one or more pairs of ultrasonic transducers are attached to a pipe (or a spool piece attached to a pipeline), where each transducer pair includes a transducer located upstream and a transducer located downstream from each other. Each transducer, when energized, transmits an ultrasonic beam or signal (e.g., a sound wave) along an ultrasonic path through the flowing fluid that is received by and is detected by the other transducer of the pair. The path velocity (i.e., path or chord velocity (Vp)) of the fluid averaged along an ultrasonic path can be determined as a function of the transit time differential between the transit time of an ultrasonic signal traveling along the ultrasonic path from the downstream transducer upstream to the upstream transducer against the fluid flow direction, and the transit time of an ultrasonic signal traveling along the ultrasonic path from the upstream transducer downstream to the downstream transducer along the fluid flow direction.
There are two different measurement principles used in known transit time ultrasonic flowmeters. A first type of ultrasonic flowmeter is a direct-path type that implements direct measuring using crossed paths between transducer (sensor) pairs, where there are no reflectors used. The ultrasonic transmitter and receiver for the direct-path type ultrasonic flowmeter are always located diagonally on opposite sides of the meter pipe wall. A second type of ultrasonic flowmeter is a reflective path type that implements indirect measuring paths using an ultrasonic reflector on the meter pipe wall opposite to the transducer pair to reflect the ultrasonic measurement signal received from the ultrasonic transmitter to the ultrasonic receiver, where the transducer pair is located at the same side of the meter pipe wall.